1. Field of the Invention
The present invention relates generally to high heat load processes, for example, chemical vapor deposition (CVD) and more specifically to an apparatus for precisely controlling the temperature of a substrate during such high heat load processes.
2. Description of the Related Art
The current interest in chemical vapor deposition (CVD) of diamond can be traced to work in the early 1980's which showed that activating a hydrogen-hydrocarbon mixture with a hot filament could generate diamond growth rates in the 1 .mu.m/hr range. Today, growth rates for optical quality diamond films are about 2-4 .mu.m/hr. At such growth rates, fabrication of bulk diamond windows on the order of 10 cm in diameter and 1 mm thick can require greater than 500 hours or nearly 21 days.
Most CVD processes occur at high temperatures on the order of about 200.degree.-2000.degree. C. CVD processes include oxygen-acetylene flame CVD processes, atmospheric pressure plasma CVD processes, lower heat flux CVD processes, microwave assisted CVD processes and filament assisted CVD processes. In the field of diamond growth by flame CVD, for example, a diamond substrate or a non-diamond substrate is held within an oxygen-acetylene flame in order to promote the deposition of diamond on the substrate in a hydrogen and hydrocarbon rich atmosphere. The temperature of the substrate is regulated between 200.degree.-2000.degree. C. while the diamond grows during the CVD process. Typically, the substrate temperature is difficult to control with precision. Due to the high heat loads used, for example, in excess of 1 kW/cm.sup.2, it is difficult to control with precision the temperatures between 200.degree.-2000.degree. C. at the substrate or at the substrate mount and it is difficult to prevent the substrate from overheating. The lack of precise temperature control at the substrate or at the substrate mount during high heat load processes such as the chemical vapor deposition of, for example, diamond is due to a deficiency in the control over thermal heat conduction carrying heat away from the substrate attached to the substrate mount.
One of the critical experimental parameters affecting the quality, for example, the growth of graphite instead of diamond, is the substrate temperature, which, in the case of homoepitaxy, is the seed crystal temperature. In CVD processes, a substrate mount rod with a pre-attached diamond substrate, seed crystal diamond substrate or non-diamond substrate may be used. Alternatively, no such pre-attached substrate mount need be used, in which case, the bare upper surface of the substrate mount rod is itself referred to as the substrate. Most CVD reactors operate between 200.degree.-2000.degree. C., wherein such high heat loads require active cooling of the substrate in order to maintain a desired temperature. The substrate temperature can be adjusted by varying the CVD reactor's power level; however, varying the power level tends to change the gas phase deposition chemistry within the reactor, thus altering the growth conditions and the material properties of the diamond grown.
Changing the coolant flow rate to the substrate mount housing (the heat sink) does not give much dynamic precision control over the desired temperature range, especially in high growth rate processes employing an oxygen-acetylene torch or an atmospheric pressure plasma torch. Nor does the use of other types of heat sinks provide any better precision control of the substrate temperature. Other types of heat sinks include spray coolers, closed loop heat transfer oil systems and heat pipes which can all be substituted for a fluid cooled substrate mount housing, for example, a water cooled copper housing. While several groups have tried inserting thermal resistors between a water cooled housing (substrate mount holder) and the substrate mount, such an apparatus does not provide dynamic precision control during high heat load processes such as CVD. In addition, such techniques do not provide precision temperature control with uniformity over large area substrates.
With the substrate attached to a molybdenum rod (substrate mount), a molybdenum rod threaded into a water cooled housing (substrate mount holder) has been used to control the temperature at the substrate during CVD. The rod can be screwed into or out of the water cooled housing as needed to control the magnitude of heat transfer away from the substrate, which, in turn, controls the temperature of the substrate during CVD. While varying the degree of exposure of the molybdenum rod out of the water cooled housing alters the resistance of the thermal pathway between the substrate and the water cooled housing, this technique cannot control substrate temperature with precision or control substrate temperature with uniformity over a large area substrate. This technique uses the mechanical motion of an object (substrate mount rod), typically, at 900.degree.-1400.degree. C., wherein precise temperature control is difficult to maintain. For example, after about one hour of growth in air, using a threaded molybdenum rod as a substrate mount rod, the molybdenum oxidizes to molybdenum oxide on the rod's threads. Subsequently, it becomes difficult or impossible to rotate the molybdenum rod either into or out of the water cooled housing. As a result, the temperature of the substrate and the substrate mount cannot be controlled with the precision desired.